Spatiotemporal pattern and regional characteristics of visibility in China during 1976-2010

Degradation in visibility occurs extensively across China, creating the urgent need to better understand visibility patterns and characteristics in China. Using daily observation data from the U.S. National Climatic Data Center (NCDC) during 1976-2010, the spatial distribution and temporal trends of visibility in China, as well as seasonal and regional characteristics, were determined. The distribution of visibility was characterized by a longitudinal gradient between west and east China. The northwest of China generally experienced consistent high visibility of > 20 km, while central and eastern China experienced visibility degradation and increase in haziness. The 133 stations with significant downward trends in visibility were clustered around central-east China and the average reduction rate was -1.65 km/decade (from 18.5 km). The 96 stations with increasing trends of visibility were mostly on the Qinghai-Tibet Plateau and Xinjiang and were closely related to a reduction in local sand storms. The average visibility in 12 key regions in China varied between 9.9-17.8 km (except for 23.9 km in Urumqi), which was lower than the national average of 18.7 km. The Taiyuan-Hohhot, Guanzhong, Yangtze River Delta, Pearl River Delta and Fuzhou-Xiamen areas all encountered significant decreases in visibility, while minor increases were observed in Beijing-Tianjin-Hebei and the Cheng-Yu region. At most stations, the worst visibility was measured in winter, but the season with best visibility varied between regions. A comparison of visibility trends with the emission trends of various pollutants revealed that SO2 emissions dominated the visibility degradation in eastern China with NO (x) and volatile organic compounds playing ascending roles during the 2000s.